Influence of geological thinking on Darwin

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Influence of geological thinking on Darwin

• By time of The Beagle voyage idea that Earth
  was young was being challenged.
• Opposition based on principle of
  Uniformitarianism.
• Idea that geological processes happening today
  are the same as have operated in the past.

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Influence of geological thinking on Darwin

• Uniformitarianism contrasted with Catastrophism
  which proposed that current geological formations
  had resulted from catastrophic events (such as
  biblical flood) which occurred on scale unknown
  today.
• Uniformitarianism first proposed by James Hutton
  and championed by Charles Lyell.

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Influence of geological thinking on Darwin

• Hutton and Lyell inferred Earth must be very old
  based on measurements of rate of ongoing rock
  forming processes (e.g. deposition of mud and
  sand).
• These developments in geology focused Darwin on
  the potential importance of gradual change in
  shaping structures.

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Darwinian Natural SelectionArtificial Selection

• Artificial Selection. Humans have selectively
  bred for desirable traits in domestic animals and
  plants for millenia.
• Process has produced our crop plants, garden
  plants, pets, and domestic animals.
• Darwin closely studied pigeon breeding.

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Artificial Selection

• Cauliflower, broccoli, kale, brussels sprouts all
  descended from wild cabbage.
• All these crops can be crossed and produce
  fertile offspring.
• Cauliflower edible bit is the inflorescence or
  flower stalk.

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Artificial Selection

• Cauliflower has large dense infloresence. This
  results from mutant loss of function alleles of
  two genes that affect flower structure and
  infloresence density.

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Artificial Selection

• Early farmers choosing among their crops selected
  those with largest infloresences. Process has
  resulted in cauliflowers that are homozygous for
  both loss of function alleles.

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Evolution by Natural Selection

• Darwin envisaged process similar to artificial
  selection that had produced organisms we see
  today. He called it Natural Selection.

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Evolution by Natural Selection

• Darwin proposed evolution the inevitable outcome
  of 4 postulates
• 1. There is variation in populations.
  Individuals within populations differ.
• 2. Variation is heritable.

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Evolution by Natural Selection

• 3. In every generation some organisms are more
  successful at surviving and reproducing than
  other. Differential reproductive success.
• 4. Survival and reproduction are not random, but
  are related to variation among individuals.
  Organisms with best characteristics are
  naturally selected.

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Evolution by Natural Selection

• If 4 postulates are true then the population will
  change from one generation to the next.
• Evolution will occur.

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Evolution by Natural Selection

• Darwinian fitness ability of an organism to
  survive and reproduce in its environment.
• Fitness measured relative to others of its species

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Evolution by Natural Selection

• Adaptation is a characteristic or trait of an
  organism that increases its fitness relative to
  individuals that do not possess it.

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Testing the postulates

• Evolution of beak shape in Darwins Finches.
• Peter and Rosemary Grants (and colleagues) work
  on Medium Ground Finches Geospiza fortis
• On Daphne Major since 1973.

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Evolution of beak shape in Darwins Finches.

• Postulate 1. Is the population variable?
• Finches vary in beak length, beak depth, beak
  width, wing length and tail length.

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Evolution of beak shape in Darwins Finches.

• Postulate 2 Is variation among individuals
  heritable?
• Variation can be a result of environmental
  effects.
• Heritability proportion of the variation in a
  trait in a population that is due to variation in
  genes.

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Evolution of beak shape in Darwins Finches.

• Peter Boag compared average beak depth of parents
  with that of their adult offspring.
• Strong relationship between offspring and parent
  beak depths.

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FIG 3.7
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Evolution of beak shape in Darwins Finches.

• Postulate 3 Do individuals differ in their
  success at survival and reproduction?
• 1977 drought 84 of G. fortis individuals died,
  most from starvation. In two other droughts 19
  and 25 of the population died.

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Evolution of beak shape in Darwins Finches.

• Seed densities declined rapidly during drought
  and the small soft seeds were consumed first.
• Average size and hardness of remaining seeds
  increased over the course of the drought.

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FIG 3.8b
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FIG 3.8A
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Fig 3.8c
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Evolution of beak shape in Darwins Finches.

• Postulate 4 Are survival and reproduction
  nonrandom?
• Do those who survive and reproduce have different
  characteristics than those that dont?

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Evolution of beak shape in Darwins Finches.

• As drought progressed small soft seeds
  disappeared and large, hard Tribulus seeds became
  a key food item.
• Only birds with deep, narrow beaks could open
  them.

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Evolution of beak shape in Darwins Finches.

• At end of the 1977 drought the average survivor
  had a deeper beak than the average non-survivor
  and also a larger body size.

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FIG 3.9
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Did the population evolve?

• Chicks hatched in 1978 had deeper beaks on
  average than those hatched in 1976.
• Population evolved.

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Fig 3.10
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Evolution of beak shape in Darwins Finches.

• Variation in weather from year to year on Daphne
  Major over 30 years has led to variation in the
  traits that are favored by selection.
• Population has evolved over time.

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Fig 3.11 A
Over the course of 30 years (1970 to 2000) beak
size evolved. Rose sharply during drought (red
line) then declined to pre-drought dimensions.
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Fig 3.11 B
Over same 30-year period birds evolved more
pointed beaks and (next slide) significantly
smaller body size.
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Fig 3.11 C
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The nature of Natural Selection

• Many misconceptions about how selection operates
  and evolution occurs.
• Points to remember about natural selection

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Natural selection acts on individuals, but its
effects accumulate in populations

• Individual finches live or die during a drought
  (the selection event).
• But change occurs in the characteristics of the
  population, not in individuals.

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Natural selection acts on individuals, but its
effects accumulate in populations

• During drought individual finchs beaks did not
  change, but average beak dimensions changed
  because more small-beaked birds died than
  large-beaked birds.

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Evolution causes changes in allele frequencies

• Evolution only occurs when traits have a genetic
  basis.
• If beak dimensions were environmentally induced,
  no evolution could take place. After drought,
  frequencies of phenotypes in next generation
  might have been the same as before.

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Natural selection does not plan ahead.

• Each generation is result of selection by
  environmental conditions of the previous
  generation.
• Evolution always one generation behind
  environmental changes.

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New traits evolve even though selection acts on
existing traits.

• This occurs because
• 1. mutation produces new alleles.
• 2. In sexually reproducing organisms meiosis and
  fertilization recombine existing alleles to
  produce new genotypes.

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New traits evolve even though selection acts on
existing traits.

• Artificial selection for oil content in corn.
• After 60 generations oil levels were well above
  starting values.

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Fig 3.12
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New traits evolve even though selection acts on
existing traits.

• Natural selection can also modify existing
  features over time for a new purpose e.g. Pandas
  thumb.
• Trait used in novel way and eventually developed
  into a new structure referred to as a
  preadaptation. This does NOT mean there is
  pre-planning by natural selection.

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Natural selection does not produce perfect
solutions

• Pandas thumb not a perfect solution.

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Natural selection does not produce perfect
solutions

• On Daphne Major during drought finches with
  narrow beaks survived better than those with
  wider bills.
• At end of drought, however, selection for deeper
  bills and bigger body size resulted in wider
  beaks even through deeper narrower beaks would
  have been a better solution.
• Presumably same genes control all three traits.
  And solution is not perfect.

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Natural selection does not produce perfect
solutions

• Similarly, many characteristics of organisms are
  the result of compromises between different
  selection pressures.
• The wings of various species of auk (seabirds
  that dive and swim) are a compromise between the
  need to fly (i.e. swim) underwater and in the
  air, two very different media.

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Little Auk
polar.alaskapacific.edu/aharding/images/Littl...
Razorbill
http//media-2.web.britannica.com/eb-media/16/2601
6-004-13D8FA4C.jpg
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Natural selection is nonrandom, but not
progressive

• There is no goal of natural selection.
• Evolution makes organisms better adapted to their
  environments, but there is no trend towards being
  more advanced.
• E.g. Tapeworms have no digestive tract. They are
  simpler than their ancestors.

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Selection does not act for the good of the
species

• Apparently altruistic acts (e.g. giving an alarm
  call) are favored because they enhance relatives
  survival.
• Infanticide in lions benefits individual male
  lions not the species as a whole.